Synchronizing system for sawtooth wave generators



Dec. 20, 1949 D. E. sUNsTElN 24929918 SYNCHRONIZING SYSTEM FOR SAW TOOTHWAVE GENERTORS Filed Nov. 11, 1944 2 Sheets-Sheet 2 Z/ l GAS FILLEDKap/d 1 2 Patented Dec. 20, 1949 SYNCHRONEZING SYSTEM FOR SAW- TOOTHJ'VAVE GENERATRS David E. Sunstein, Elkins Park, Pa., assigner to PhilcoCorporation, Philadelphia, Pa., a corporation of PennsylvaniaApplication November 11, 1944, Serial No. 563,027

(Cl. Z50-36) 5 Claims.

The present invention relates to an automatically synchronized saw toothgenerator and more particularly to such generator for use as a time axisgenerator or sweep control for an oscilloscope. For certain applicationsof the oscilloscope the saw tooth generator is of the type which issynchronized with another operation as in the case of televisionsystems, vibration, acoustic, and speech investigations and studies.Where synchronization is to occur in response to a variable frequencyinput, generally it has been necessary to provide a number of manualcontrols to adjust the operation of the saw tooth generator.Considerable skill is required in adjusting such manual controls, whichmay be rather large in number, in order to provide the desiredoperation. It, therefore, would be desirable to provide a saw toothgenerator which would be synchronized automatically to the variations infrequency of the initiating voltage, and which has a wide range ofoperation over the desired band of frequencies and which provides a sawtooth wave having an amplitude substantially independent of thefrequency.

In accordance with the present invention a saw tooth generator isprovided with a frequency to-voltage translator responding` to thecontrol of input frequency which is used to control the rate ofoperation of an either variable discharging device or a variablecharging device for an energy storage circuit. The frequency-to-voltagetranslator responds to the change in frequency to modify the operationof the charging or discharging device so that the saw tooth wave ismaintained at substantially constant amplitude throughout the range ofoperation over a band of frequencies.

The frequency-to-voltage translator controls the rate of charging ordischarging of the capacitor or energy storage circuit on the basis ofsev-v eral precedingl cycles of the input control frequency. If it isdesired to have an extremely rapid rate of response to the change of therate of the input frequency, the frequency-to-voltage translator may bemodified to measure the period of one previous cycle and to control thecharging or discharging on that basis. Such an arrangement may also becombined with an automatic amplitude control responsive to the amplitudeof the output of the `saw tooth wave which in conjunction with thefrequency-to-voltage translator operates to control the charging ordischarging circuit for the capacitor.

It, therefore, is an object of the present in. vention to provide animproved timing axis generator for an oscilloscope which will provide anoutput wave or signal of substantially constant amplitude independentlyof the frequency of operation.

Another object of the present invention is to provide an improved sawtooth generator Which is automatically synchronized with a varyingfrequency control signal, such as speech, music, or the like.

Still another object of the present invention is to provide an improvedcircuit which may be used as a frequency divider.

Other and further objects of the present in venton subsequently willbecome apparent from the following description taken in connection withthe accompanying drawings in which Figure l is a block diagram of oneembodiment of the present invention;

Figure 2 is a block diagram of another embodiment of the presentinvention;

Figure 3 is the embodiment shown in Figure 1 illustrating certaincircuit details;

Figure 4 is a block diagram of still another embodiment simiiar to thatshown in Figure 1;

Figure 5 shows certain details of the embodiment of Figure 4;

Figure 6 is a diagram of an embodiment having a more rapid rate ofresponse to frequency change;

Figure 7 is a block diagram of still another embodiment; and

Figure 8 is a diagram showing certain circuit details of the embodimentof Figure 7.

The block diagram in Figure 1 shows a source of signal energy i Iarranged to control the operation of a rapid charging device I2 having asuitable source of energy such as the battery I3. The rapid chargingdevice supplies energy to an energy storage circuit in the form of acapacitor I4 which is connected to a pair of output terminals I5.

f The source of energy I3 and the capacitor I4 are each connected toground. The capacitor I4 is discharged by a variable discharging deviceI6 the rate of operation of which is controlled by a 40frequency-to-voltage translator Il, receiving energy from the signalsource II. In this circuit arrangement the capacitor It is rapidlycharged to a certain voltage value by the rapid charging device I2. Uponreaching this predetermined value the variable discharging device Itreduces the charge across the capacitor I4 to a predetermined value in acertain time interval. The time interval is determined by the rate ofoperation of the variable discharging device It Which is controlled bythe frequency-to-voltage translator I'I. The variable discharging deviceI6 preferably is of a type which discharges the capacitor at a constantrate for any given value of the rate of discharge as determined by theaction of the frequency-to-voltage translator il. When the capacitor I4has been discharged or partially discharged the cycle is repeated. Thisoperation produces a saw tooth output voltage across the terminals I5having a steep or substantially perpendicular wave front followed by asubstantially The capacitor is is connectedto a rapid dis- ,v

charging' device I8 which rapidly dissipatesthe charge across thecapacitor I,4 so that the discharging portion of the saw toth waveappearing across the terminals I5 is substantially perpendicular. Thecapacitor I4 is charged to a predetermined*voltage value by a variablecharging device lil connected in series with the 4voltage source i3rsothatrth'e wavefront of the saw tooth generated by the capacitor has asub*` stantially constant slope for any constant irequency Iof thesignalgenerator II. vThewoutput wave obtained by blockudiagram inFigure@ therefore is a reversal of the saw tooth wave obtained by theblock diagram of Figure 1. l 4

i To illustrate the manner in which the block diagram of lFigure 1 maybe constructed there is shown in Figure 3 a signal generator Il which isconnected to a square wavegenerator 2i coupled through a blockingcapacitorr22 to the grid of an electric valve 23. The electric valveI2.3 coniprises a tube lof the type` having within its envelope a gas oran io-ninable medium as yis the case in Thyratron tubes. The gas tube 23constitutes thetube used in the rapid charging device i2 and its anodeis connected to the posin tive terminal of a suitable source of voltagesuch as a source of voltage I3 as shown in Figure 1. The grid of the gastube 23 has a grid resistor 24 connected to an adjustable contact on aresistor 25 vwhich is connected between ground and a suitable source ofpositive biasing voltage The Tliyratron 23 has its vcathode connected tothe capacitor i t whichin turn is provided with a current limitingresistor n26 having one terminal connected to ground. The circuit thusfar described therefore constitutes a 'rapid charging circuit for thecapacitor I4. The capacitor I4 is connected between 'the/ anode andcathode of a vacuum tube `2l which preferably is of the pentode typewhich has the characteristic of maintaining substantially constant anodecurrent even though the voltage between the anode and cathode variesover a substantial range. The pentode 21 is used todischarge thecapacitor iii, and the rate of operation ofthe pentode 2l is controlledin accordance with the voltage determined by the frequency-to-voltagetranslator I? of Figure 1. The `suppressor grid of the vacuum tube 21 isconnected to ground. The screen grid of the vacuum tube 21 is connectedto a suitable source of potential, preferably through an 1in-bypassedresistor 2G. The vcontrol grid of the vacuum tube 2T is connected to aswitch arm 28 adapted to contact a plurality of contact pointsveachhconnected to different portions of a resistor 29. comprising aresistor 3I and a capacitor 32 connested between ground and the squarewave generator 2i integrates the output of the square wave generator t`osupply an integrated output to a diode rectifier 33 having its cathodegrounded. The anode of the diode rectifier 33 therefore is connectedbetween the common juncture of the resistor SI and the capacitor 32. Thediode rectiyri'er 33 operates t'o form fa 'unidirectional voltage bias"across the capacitor 32, which is inversely An integrating networkproportional to the frequency supplied by the signal generator'A Ii atthe synchronous input terminals of the square wave generator E I. Theunidirectional voltage thus generated is filtered by a resistor 34 and acapacitor 35 to supply voltage to the voltage divider resistor 2Q. *3yrnv/"ement of the switch arm 28, a certain portion of the direct currentvoltage is supplied to the control grid of the variable discharge tube2. When the voltage supplied to the control grid oi the vacuum tube 2'!`is a large portion of the voltage appearing across the resistor 2t', acertain step-down ratio is obtained between the irequency of the signalgenerator Ii and the frequency of the saw tooth voltage appearing acrossthe outputterminals Thus the arrangement Figure 3 by operation of theswitch 28 may beused as a frequency divider. The circuit elementsassociated with the diode comprise the frequency-tolvoltagetranslator Ilin the diagram of kiigure 1t `As the frequency of the signal voltage ilchanges, the direct current cias appearing across tlie capacitor 32 ischanged so that the voltage supplied to the discharging vacuum tube 2 ischanged therefore modifying the rate of discharge of the capacitor i4 sothat the irequency of the output saw tooth wave appearing across theterminals yI5 is rapidly brought into synchronism with the stabilizedchanged irethrough the variablev discharge device I6.

4 quencynoi the signal generator I I.

V1n order to increase the range of operation of the system showninyigures 1 and 3 certain modications may be introduced as illustratedby the block diagram of Figure 4. In this arrangement the various partsyhavebe'en given reference characters corresponding to the same parts inFigures 1 and 3 fromlwhich it will be seen that the output terminals I5appear across the series circuit comprising the capacitor I4 andthecurrent limiting resistor 2li. The variable discharging device Iiiuisdirectly connected across the caps citorl I4. The operation'oi 4thedischarging device I6 is controlled. by the Vdifference between twosignalsq One of these signals is derived from the frequencyfto-voltag'etranslator I'I, and the other signal is derived 4from the currentlimiting resistor 25. The second signal is proportional to the directcurrent component of current owing The voltage component appearingacross the resistor 26 is supplied to a low pass lter 3l the output ofwhichis connected to a direct current amplifier 38. The variable vdischarge I6 also receives a voltage component vfrom thefrequency-to-voltage translatorl'l from which is subtracted the directcurrent component supplied by amplifier 38. This produces a Icurrentdegeneration which insures a greater rangev of'loperation than isobtainable by the circuit arrangement of Figure 3 by overcoming anyinherent lnonlinearity between the rate of discharge of the vacuum tube27 and the signal supplied` from the frequency-to-voltage translator Il.Thus there is insured that the currevnttdraw'n by the'discharge tube 2lwill bear a nearly linear relationship to the voltage sup plied to itscontrol gri'd over a very wide range of voltages supplied thereto, ascontrasted to a relatively narrower 'range capable of being handled bythe circuit 'arrangement in Figure 3. i One manner in which themodification of Jigure '4 may b'eapplied to the circuit arrangementshownin Figure3 is illustratedin Figure 5. The adjustable contact 28 for'the resistor 2e is connectedvto the control grid of the Variabledischarge tub'e 2,1. v'The 4screen grdis supplied from a suitable sourceof potential through a resistor l 39 which is by-passed to ground by acapacitor 4I. The voltage supplied to the screen grid of the vacuum tube21 is modified by the direct current component obtained across thecurrent limiting resistor 26 which is connected to -a lowpass filtercomprising a resistor 42 and a capacitor 43. The common juncture betweenthe resistor 42 and the capacitor 43 is connected to a grid of a vacuumtube 44 having its anode connected to the screen grid of the vacuum tube21. The cathode of the vacuum tube 44 is connected through a biasingresistor 45 to ground. The vacuum tube 44 operates as a direct currentamplier with the resistor 39 operating as a plate or load resistor forthe vacuum tube 44. The alternating current components are by-passed bythe capacitor 4I. The voltage appearing at the anode of the vacuum tube44 therefore is the voltage which is supplied to the screen grid of thevacuum tube 21. If it is assumed that the signal source II is increasedin frequency, the integrator circuit including the resistor 3| and thecapacitor 32 will cause the diode rectifier 33 to operate to reduce thenegative bias applied to the grid of the variable discharge vacuum tube21. This produces an increase in the rate of discharge of the capacitorand in the current flow through the tube 21 so that an increased voltagedrop is produced across the current limiting resistor 26. This voltagedrop across the resistor 26 is amplified by the direct currentamplifying tube 44 to lower the screen grid voltage of the dischargetube A21 thereby tending to reduce the current drawn by that dischargetube. This tendency therefore tends to maintain more nearly linear themodified rate of discharge of the capacitor I4 for the changed signalfrequency. The circuit therefore obviates the frequency range limitationobtained by the circuit arrangement in Figure 3 which has -an inherentnonlinearlty above a certain frequency range.

In the preceding arrangements shown in detail in Figures 3 and 5 therate of discharge of the vacuum tube 21 was controlled in accordancewith the average of several preceding cycles of energy obtained from thesignal source. It, therefore, will be appreciated that there is -acertain time delay between the change of frequency of the signal sourceand the time when the variabe discharge device reaches synchronism so asto produce a saw tooth wave of uniform amplitude in synchronism with thesignal frequency. If it is desired to have an arrangement whereby theoutput wave responds very rapidly to any change of the frequency of theinput wave, certain modifications may be made as are illustrated inFigure 6. A pulse generator 46 is arranged to be energized from thesignal source l I; and the pulse generator, which preferably is of thetype having an output wave of the peaked variety, supplies this wave tothe grid of a Thyratron 41. The Thyratron 41 is connected, between thesuitable source of anode potential and ground, in series with acapacitor 48 and a current limiting resistor 49. The peaked pulses ofshort duration impressed upon the grid of the Thyratron 41 produceionization so as to bring about the charging of the capacitor 48 insynchronism with such pulses. The capacitor 48 is discharged at asubstantially constant rate, either by a resistor or by a pentode vacuumtube I having its anode connected to the capacitor 48 and its cathodeconnected to ground. The pentode 5I has its suppressor grid connected toground, its screen grid connected to a suitable source of potential, andits control grid is connected to the cathode. This produces a saw toothwave which is fed through a coupling capacitor 52 to a diode vacuum tube53. A resistor 54 is connected to the capacitor 52 so that the capacitorand resistor combina tion constitute a differentiating circuit which produces at the anode of the diode rectifier 53 a.r peaked wave havingamplitude inversely propor-rv tional to the incoming frequency. Thispeaked rwave supplied to the diode rectifier 53 is utilized to charge acapacitor '55. The capacitor 55, however, is so arranged as to have beendischarged at the time that the capacitor 48 was being charged. This isobtained by a Thyratron 56 and a biasing source of voltage 51 connectedin parallel with the capacitor 55. The control electrode or grid of theThyratron 56 receives energy from the common juncture between thecapacitor 48 and the current limiting resistor 49 so that the "Thyratron56 operates to discharge the capacitor 55 during each cycle or pulsecorresponding to the frequency of the signal voltage I I. Since thecapacitor 55 is arranged to be discharged once during each cycle, eachsucceeding charge is inversely proportional to the incoming frequency.The voltage appearing across the capacitor l55 therefore may be suppliedto a phase inverter 58 so as to control the variable discharging deviceI6. The phase inverter 58, which may comprise a single stage amplifier,is provided in order to make the grid of the discharge tube of thevariable discharging device I6 more negativev as the input frequency ofthe signal generator Il is decreased. Such an arrangement will controlthe bias of the variable discharging device I6 so as to adjust the rateof discharge to the incoming frequency `within a time intervalrepresented by one cycle of the incoming frequency, thus insuring rapidadjustment of the output frequency and amplitude appearing across theterminals I5 with respect to the change in frequency of the signalvoltage Il.

While the systems heretofore described have the inherent characteristicof maintaining a constant amplitude saw tooth output independent ofchanges in the signal frequency, oversensitivity of frequency-to-voltagetranslator I1 or of variable discharging device I6 or of variablecharging device I9 will cause the amplitude to fall off with an increaseof frequency, and undersensitivity will have the opposite effect.Therefore, in some cases it may be desirable to introduce an actionwhich has inherently the characteristic of tending to produce a slightlyreduced amplitude of saw tooth wave with an increase in the operatingfrequency. Such an arrangement is illustrated in Figure '1 which inaddition to the previous components illustrated by the block diagrams inFigures 1 and 2 utilizes an automatic amplitude control 59 arranged tobe energized directly from the capacitor I4. A component derived fromthe automatic amplitude control 59 is combined with a component ofvoltage derived from the frequency-to-voltage translator I1 by a voltageadder 6|. The voltage adder 6I is arranged to control the operation ofthe variable discharging device I6. The manner in which the automaticamplitude control serves to supply the voltage component is illustratedin Figure 8 where one terminal of the capacitor I4 is connected througha coupling capacitor 62 to the grid of a vacuum tube 63 which serves asan isolation stage. The cathode of the vacuum tube 63 is connected toone of the output terminals I5, the

other output terminal l5 being connected tol ground. A voltage dividercomprising resistors 64 and 65 is connected between ground and thecathode oi the vacuum tube 63. The grid of the vacuum tube 63 isconnected to series resistors 66 and 61 having their common junctureby'- passed by a capacitor 68 to the cathode of the vacuum tube 63. Theother terminal of the re sistor 61 is connected to thecommon juncture ofthe resistors 64 and 65. The resistor 66 corresponds to a grid resistor,and the resistor 61 and the capacitor 68 constitute a filter circuit.The isolation stage thus described operates to reduce' any load on thecapacitor I4 since it is desiredl to make such load negligible so as tomaintain the discharge rate of the capacitor linear and' controllable bytube 21. The isolation stage therefore permits the' net resistivecomponent of the impedance across theA capacitor I4 to be keptl at arelatively high value so' that the' size of the capacitor may berelatively small even at low frequencies.

The common juncture between the' resistors' 64 and 65 is capacitivelycoupled by a capacitor' 68' to the anode of a diode re'c'tier 69. The'anode' o'f the diode rectifier is provided with a grounded resistor 1l,and the voltage developed thereacr'oss is supplied to a lter circuitcomprising resistors' 12 and 13 and by-pass capacitors 14 and 15. Theoutput of the filter isv connected to a resistor 16 which also receivesthe voltage component frorn the frequency-toevoltage translator l1. The'juncture of resistors' 13r andl 1B is connected to the grid of dischargetube 21 so thatv the discharge tube 21 has its rate ofdischargecontrolledby theA i sum of two voltage components so as to'give the desired operational characteristic. Thus resistor 16 workingwith resistors 1|', 12 and 13`cons'titute the voltage adder 6I.

While for the purpose o'f describing and illusi-y trating the presentinvention, certain specic embodiments have been shown in the drawings,itis t0 be understood that the invention is not to be limited therebysince such variations' in the circuit arrangements andin the'instrumentalitiesf employed are contemplated as may be commen` suratewith the spirit and scope ofthe invention deiined in the followingclaims;

This invention is hereby claimed as follows:

1 The combination comprising a source ofT unidirectional voltage, acapacitor, means for charging said capacitor from saidsource of volt-rage, i eens for discharging said capacitor, vone of sai means having arapid `rate of operation;

the other of said means having a variable rate 5.5

of operation, a source of signal volt'ag'e'ior` controlling theoperation of theV iirst oneof said? means, means for deriving a rstcontrol voltagel from said signal source, the' amplitude of said5rcontrol voltage depending upon the frequency oi` said signal, means forderiving ase'co'nd'con'trolj voltage from the current through eitherofvv saidl first two means, and means for applying bothL of said controlvoltages tothe meanshaving: a variable rate of operation .so` asto'control-thelatter.

2. The combination comprising asource' ofi unidirectional current', acapacitor; means for'v charging said capacitor fromsaidsourceoff volt-`age, means for discharging said'- capacitor, one' of said means having arapid' rate of operation;

8i th'e'- other' of said means having a rate of operation`l which may bevaried, a sources of signal voltage' for controlling the frequency ofoperation cf one' of said means, means responsive to the voltage`diference appearing across said capacitor for producing a controlvoltage, means for deriving a' second control voltage in accordancewiththe frequency of said signal voltage, and means for combining saidcontrol voltages for varying the rate of operation of said second means.

3. The' combination' comprising a source of unidirectional' current, anenergy storage circuit, means for charging said circuit from said sourceof voltage,y means for discharging said circuit, one of said' meanshaving a rapid rate of operation, the other' of said means having a rateof operation which' may be varied, a source of signal voltage forcontrolling the frequency of operation' of one of said means, meansresponsive to' the` voltage difference appearing across said circuit forproducing a control voltage, means 'for' driving ai secondcontrolvoltage in accordance withI the frequency of said signal voltage, andmeans for combining said control voltages for varying the' rate' ofoperation of said second means.

41The' combination! comprising a source of unidirectional voltage, anenergy storage device, means' forchaiging said' device from said sourceo'i voltage, means for' discharging said device, onev of said meanshaving a rapid rate of operation, the other of said means having a rateof operationr which maybe varied, a source of sig nal' voltage' forcontrolling the frequency of operation of 'the rst one of said means,means for deriving a rst control' voltage from said signal source, the'amplitude off said control voltagev depending upon' the frequency ofsaid signal, means for deriving a"4 second' control' voltage from thecurrent' through` either of said first two means,

and means for applying both of said controlv stantially constant rate, asecond capacitor,

means responsiveI to the voltage across said rst capacitor'for chargingsaid second capacitor in inversev proportion' to the' frequency of saidpulses, means' for discharging said' second capacitorCduring' theoccurrence of each pulse, and

means'for deriving ari output' voltage `from acrosssaid'secondlcapacitor.

' DAV ID E. SUNSTEIN.

REFERENCES CITED lThe following references are of record in the le' ofthis patent:

UNITED STATES PATENTS Number Name Date 2,265,290" Knick-y Dec. 9, 19412,266,516 Russen Dec. 16, 1941 2,420,303 De France May 13, 19472'j448g069 Ames, Jr. Aug. 31, 1948- 21448Q0'70" Sunstein Aug. 31, 1948

